The present invention relates to a process for the preparation of polyamines of the diphenylmethane series.
Polyamines of the diphenylmethane series are understood as being compounds and compound mixtures which correspond to the following general formula:
                wherein:                    x represents a number from 2 to n, and            n represents a natural number >2.                        
The continuous, discontinuous or semi-continuous preparation of polyamines of the diphenylmethane series, also called MDA in the following, is described in numerous patents and publications (see e.g. H. J. Twitchett, Chem. Soc. Rev. 3(2), 209 (1974), M. V. Moore in: Kirk-Othmer Encycl. Chem. Technol., 3rd ed., New York, 2, 338-348 (1978)). The preparation is conventionally carried out by the reaction of aniline and formaldehyde, in the presence of acid catalysts. HCl is conventionally employed as the acid catalyst. According to the prior art, the acid catalyst is neutralized, and therefore used up, by addition of a base at the end of the process and before the final working up steps (such as, for example, removal of excess aniline by distillation).
Generally, it is known that acid condensation of aromatic amines and formaldehyde to yield polyamines of the diphenylmethane series proceeds in several reaction steps. These are explained, by way of example, in the following equation:



In the absence of an acid catalyst, formaldehyde first undergoes condensation with aniline to yield so-called aminal and water. The rearrangement that results in MDA takes place under acid catalysis in a first step to yield para- and ortho-aminobenzylaniline. The aminobenzylanilines then rearrange in a second step to give MDA. The main products of the acid-catalysed reaction of aniline and formaldehyde are the diamine 4,4′-MDA, its position isomers 2,4′-MDA and 2,2′-MDA, and higher homologous polyamines of the diphenylmethane series.
In the presence of an acid catalyst, the aminobenzylanilines are formed directly from aniline and formaldehyde. These then react further to yield the MDA isomers having two rings and MDA homologues having more than two rings.
Polyisocyanates of the diphenylmethane series, called MDI herein, are prepared by phosgenation of the corresponding polyamines. The polyisocyanates of the diphenylmethane series prepared in this way contain here the various isocyanate isomers and higher homologues thereof in the same relative amounts as the polyamines from which they have been prepared.
A controlling parameter for influencing the distribution of isomers in the acid-catalysed reaction of aniline and formaldehyde is, in addition to the molar ratio of aniline and formaldehyde, the amount of acid catalyst employed in the process. In this connection, the degree of protonation of monoproton acids, such as HCl, is the molar ratio of the acid catalyst employed and the molar amount of amine functions present in the reaction mixture. In order to be able to prepare MDA with the desired distribution of isomers, sometimes considerable amounts of acid catalyst (which corresponds to a high degree of protonation), and correspondingly considerable amounts of base for the neutralization of the acid catalyst, must therefore be employed.
Thus, DE-A-1 643 449 describes the preparation of MDA having a high content of 4,4′-MDA by reaction of aniline which has been reacted with acid before being reacted with formaldehyde, such that the degree of protonation is at least 25%, preferably at least 50%, and more preferably 75 to 100%. In this context, it is necessary to employ the acid catalyst in an amount such that the reaction is carried out in a uniform phase. Both the water of reaction and the water originating from the starting substances remain in the reaction mixture.
DE-A-10 111 337 describes the preparation of MDA at a degree of protonation of <20%, however, MDA having an increased content of the 2,4′ isomer is obtained according to the invention. This process also discloses that the water of reaction and the water originating from the starting substances remain in the reaction mixture.
It is furthermore known that the distribution of isomers and homologues of the MDA can be controlled, within limits, by modifying the profiles of the dwell time and temperature. As described in EP-A-10 53 222, a high temperature is advantageous if 2,4′-MDA and 2,2′-MDA-rich types are preferably to be prepared.
It follows from the prior art, that the increasing amount of 2,4′-MDA formed at a low degree of protonation can be at least partly avoided by low temperatures during the rearrangement. A low degree of protonation, however, means a lower consumption of acid catalyst, and also a lower consumption of base required to neutralize the acid catalyst after the condensation reaction. A procedure with a low degree of protonation is, therefore, particularly economical.
However, the lower limit of the technically realizable protonation, surprisingly, is not marked by limits in the adjustment of the temperature and/or dwell time. Rather, it is found in the processes described in the prior art, that the acid reaction mixtures become heterogeneous or two-phase in the reaction below a degree of protonation of 25% (see, for example, DE-A-198 04 918). This two-phase nature leads to problems in conducting the reaction.
Therefore, an object of the present invention was to provide a simple and economical process for the preparation of polyamines of the diphenylmethane series in which the acid catalyst can be employed with degrees of protonation of <15%, without the reaction mixture demixing into a two-phase system. It is at the same time desired that polyamines having low isomer ratios of 2,4′-MDA to 4,4′-MDA be prepared.